sys/oss/: - the user-visible error strings were in the wrong category

sys/oss/: - the user-visible error strings were in the wrong category

Original commit message from CVS:

* sys/oss/Makefile.am:
* sys/oss/common.h:
* sys/oss/gstosssink.c: (gst_oss_sink_init), (gst_oss_sink_open),
(gst_oss_sink_prepare), (gst_oss_sink_unprepare):
* sys/oss/gstosssrc.c: (gst_oss_src_prepare),
(gst_oss_src_unprepare):
- the user-visible error strings were in the wrong category
- and the messages were not marked for translation
- which is actually a good thing, because they were exactly
the kind of message you would never want anyone to see
- the macros were using variables that didn't exist in the macro
arguments
- and they were obviously copied from each other and then modified
- so a common header makes sense